One embodiment of a prior art hydraulic circuit apparatus for use in construction vehicles is shown in FIG. 1.
As shown in FIG. 1, this hydraulic circuit apparatus comprises a plurality of hydraulic pumps P.sub.1, P.sub.2 and P.sub.3 for supplying pressurized fluid, and a flow control valve 6 which serves to allow the pressurized fluid in an excess amount than that required for a hydraulic circuit 5 for power steering to join the fluid flow through a hydraulic circuit A for implements. Reference characters c and d denote valves for operating the implements and are adapted for controlling the supply of pressurized fluid to hydraulic cylinders 22 and 23 for actuating the implements. Reference character e denotes a main relief valve, and T reference character a fluid tank or reservoir.
To improve the maneuverability of the implements in the above-mentioned prior art hydraulic circuit apparatus shown in FIG. 1, there is a conventional hydraulic circuit B for implements as shown in FIG. 2. The hydraulic circuit B comprises, in addition to the hydraulic circuit A for implements shown in FIG. 1, implement operating valves 7 and 8 and pressure compensating flow control valves 9 and 10 associated, respectively, therewith. In FIG. 2, reference numerals 9' and 10' indicate springs and numerals 15, 16, 17 and 18 indicate pilot pipings.
Further, in the case where the hydraulic circuit for implements is kept at a high pressure and large quantities of pressurized fluid is not required for the hydraulic circuit for implements, a prior art arrangement as shown in FIG. 3 is used. An unloading valve c is used to unload the pressurized fluid in excess of that required for the hydraulic circuit 5 for power steering upstream of the hydraulic circuit for implements A and B. This thereby reduces the power consumption for the latter.
In the case where the above-mentioned prior techniques are used, the problems are as follows.
(1) In the case where the pressure compensating flow control valves 9 and 10 are additionally provided as shown in FIG. 2, when the hydraulic circuit for implements is at its neutral position (stating more specifically, the implement operating valves 7 and 8 assume their positions 7.sub.2 and 8.sub.2, respectively), it is required to provide a fluid actuating pressure which can overcome the resilient forces of the springs 9' and 10' in the pressure compensating flow control valves 9 and 10, respectively. This fluid pressure is produced in the pilot pipings 15, 17, thus causing pressure losses corresponding to the fluid actuating pressure.
(2) In the above-mentioned prior art hydraulic circuit apparatuses shown in the FIGS. 1 and 2, in case where the flow rate of fluid under pressure delivered by the hydraulic pump P.sub.1 meets the requirements of the hydraulic circuit A or B for implements, the pressurized fluid in an excess amount or a flow rate than that supplied into the hydraulic circuit 5 for power steering becomes useless. The fluid pressure for the hydraulic circuit for implements becomes loaded on the hydraulic pump P.sub.2 or P.sub.3, thus causing a high power consumption for the latter.
(3) In the above-mentioned prior art hydraulic circuit apparatuses shown in FIGS. 1, 2 and 3, because of a high rate of fluid flow through the implement operating valves (c, d in FIG. 1, and 7, 8 in FIG. 3), there is a tendency of causing a rise in fluid temperature and generating noise due to the fluid pressure.